It is well established in neurosurgery that selected destruction of neural tissue can in many cases benefit patients with pain and other functional disorders. Typically, this invovles inserting a metal electrode with an insulated portion into the body, connecting the electrode to a source of radio frequency voltage, and heating the neural tissue at the unexposed bare tip of the electrode so as to selectively destroy a portion of nervous tissue. This is the radio frequency lesion generation technique. It was pioneered in the early fifties; see the attached review article by E. R. Cosman and B. J. Cosman, and the first papers by Dr. W. H. Sweet, Professor F. Mundinger, Wyss and Hunsperger, and Aranau, which illustrate its early development. Typically, in the most popular RF lesion generators used today (made by Radionics, Inc.) the radio frequency voltage is controlled by a manual knob on the apparatus which is turned up by the neurosurgeon at the time of surgery. The temperature of the lesion electrode is monitored by thermometric readouts at the apparatus console. Temperature control is essential in these procedures for safety, effectiveness, and consistency of lesion size; this fact being established clearly over the last three decades. Several workers and commercially available apparatus use automatic temperature control, meaning that the apparatus cycles the RF voltage up automatically until the temperature control reaches a set temperature point, at which time the radio frequency power circuit is regulated to stabilize on that set temperature. This would be referred to as full automatic temperature control. Examples of apparatus that have automatic temperature control are illustrated by the Dutch Coagrader system in the 1960's and the Fischer lesion generator systems of the 1960's and most recently in the 1980's.
There are major problems with full automatic temperature control associated with safety and controllability. Radio frequency lesion electrodes vary in size dramatically from 0.25 mm in diameter all the way up to 3 mm in diameter or more. The amount of RF power required to heat these electrode tips to a given therapeutic temperature, such as 70 or 80 degrees (.degree. C.), varies widely. Other variable factors involve tissue vascularity, impedence and various physiologic factors. A disastrous situation can take place if the RF power is increased so rapidly that the temperature swings very rapidly through a set temperature and feedback becomes unstable. The electrode tip surrounding temperature may rapidly exceed 100 degrees, the boiling point, gas will form, and one has a rapid decrease of lesioning current because of the higher impedence of the gas formation and a rapid degeneration of the temperature until the gas is absorbed. Then, with the RF level still high, boiling can reoccur, and one gets a violent oscillatory unstable heating situation. Production of gas and steam at the electrode tip can have catastrophic effects on surrounding tissue. Then the control, safety, and effectiveness of the automatic temperature control system is not reliable. One of the objectives of this patent is to describe a temperature control system, referred to as an over-temperature conrtrol, for such a heating system which both enables the operator to control a rise in temperature himself at the crucial time when instabilities could happen and yet provide stabilization at a final tip temperature which is desired. This hybrid control means has never been used before in therapeutic heating systems, whether they be radio frequency neurosurgical systems or other coagulation systems for other purposes in medicine.